Are the Effects of Use and Disuse Inherited? Part 3

MODIFICATIONS OBVIOUSLY ATTRIBUTABLE TO SELECTION.

Innumerable modifications in accordance with altered use or disuse, such as the enlarged udders of cows and goats, and the diminished lungs and livers in highly bred animals that take little exercise, can be readily and fully explained as depending on selection. As the fittest for the natural or artificial requirements will be favoured, natural or artificial selection may easily enlarge organs that are increasingly used and economize in those that are less needed. I therefore see no necessity whatever for calling in the aid of use-inheritance as Darwin does, to account for enlarged udders, or diminished lungs, or the thick arms and thin legs of canoe Indians, or the enlarged chests of mountaineers, or the diminished eyes of moles, or the lost feet of certain beetles, or the reduced wings of logger-headed ducks, or the prehensile tails of monkeys, or the displaced eyes of soles, or the altered number of teeth in plaice, or the increased fertility of domesticated animals, or the shortened legs and snouts of pigs, or the shortened intestines of tame rabbits, or the lengthened intestines of domestic cats, &c.[44] Changed habits and the requisite change of structure will usually be favoured by natural selection; for habit, as Darwin says, "almost implies that some benefit great or small is thus derived."

SIMILAR EFFECTS OF NATURAL SELECTION AND USE-INHERITANCE.

Here we perceive a difficulty which will equally trouble those who affirm use-inheritance and those who deny. Broadly speaking, the adaptive effects ascribed to use-inheritance coincide with the effects of natural selection. The individual adaptability (as shown in the thickening of skin, fur, muscle, &c., under the stimulus of friction, cold, use, &c.) is identical in kind and direction with the racial adaptability under natural selection. Consequently the alleged inheritance of the advantageous effects of use and disuse cannot readily be distinguished from the similarly beneficial effects of natural selection. The indisputable fact that natural selection imitates or simulates the beneficial effects ascribed to use-inheritance may be the chief source and explanation of a belief which may prove to be thoroughly fallacious. A similar simulation of course occurs under domestication, where natural selection is partly replaced by artificial selection of the best adapted and therefore most flourishing animals, while in disused parts panmixia or the comparative cessation of selection will aid or replace "economy of growth" in causing diminution.[45]

INFERIORITY OF SENSES IN EUROPEANS.

"The inferiority of Europeans, in comparison with savages, in eyesight and in the other senses," is attributed to "the acc.u.mulated and transmitted effect of lessened use during many generations."[46] But why may we not attribute it to the slackened and diverted action of the natural selection which keeps the senses so keen in some savage races?

SHORT-SIGHT IN WATCHMAKERS AND ENGRAVERS.

Darwin notices that watchmakers and engravers are liable to be short-sighted, and that short-sight and long-sight certainly tend to be inherited.[47] But we must be careful not to beg the question at issue by a.s.suming that the frequent heredity of short sight necessarily covers the heredity of artificially-produced short-sight. Elsewhere, however, Darwin states more decisively that "there is ground for believing that it may often originate in causes acting on the individual affected, and may thence-forward become transmissible."[48] This impression may arise (1) from the facts of ordinary heredity--the ancestral liability being excited in father and son by similar artificial habits, such as reading, and viewing objects closely as among watchmakers and engravers--or by const.i.tutional deterioration from indoor life, &c., acting upon a const.i.tutional liability of the eye to the "something like inflammation of the coats, under which they yield" and so cause shortness of sight by altering the spherical shape of the eye-ball. (2) Panmixia, or the suspension of natural selection, together with altered habits, will account for an increase of short-sight among the population generally.

(3) Long-sighted people could not work at watchmaking and engraving so comfortably and advantageously as at other occupations, and hence would be less likely to take to such callings.

LARGER HANDS OF LABOURERS' INFANTS.[49]

These are best explained as the result of natural selection and of the diminution of the hand by s.e.xual selection in the gentry. If the larger hands of labourers' infants are really due to the inherited effects of ancestral use, why does the development occur so early in life, instead of only at a corresponding period, as is the rule? During the first few years of its life, at least, the labourer's infant does no more work than the gentleman's child. Why are not the effects of this disuse inherited by the labourer's infant? If the enlargement of the infant's hand ill.u.s.trates the transference of a character gained later in life, it is evident that the transference must take place in spite of the inherited effects of disuse.

THICKENED SOLE IN INFANTS.

Darwin also attributes the thickened sole in infants, "long before birth," to "the inherited effects of pressure during a long series of generations."[50] But disuse should make the infant's sole _thin_, and it is this thinness that should be inherited. If we suppose the inheritance of the thickened soles of later life to be transferred to an earlier period, we have the anomaly of the inherited effects of disuse at that earlier period being overpowered by the untimely inheritance of the effects of use at another. On the other hand, it is clear that natural selection would favour thickened soles for walking on, and might also promote an early development which would ensure their being ready in good time for actual use; for variations in the direction of delay would be cut off, while variations in the other direction would be preserved. Anyhow, the mere transference of a character to an earlier period is no proof of use-inheritance. The real question is whether the thickened sole was gained by natural selection or by the inherited effects of pressure, and the mere transference or hastened appearance of the thickening does not in any degree solve this question. It merely excludes the effect of disuse during lifetime, and thus presents a fallacious appearance of being decisive. The thickened sole of the unborn infant, however, like the lanugo or hairy covering, is probably a result of the direct inheritance of ancestral stages of evolution, of which the embryo presents a condensed epitome. While the relative thinness of the infant's sole might be pointed to as the effect of _disuse_ during a long series of generations, its thickness is rather an ill.u.s.tration of atavism still resisting the effects of long-continued disuse. There is nothing to show that the inheritable portion of the full original thickness was not gained by natural selection rather than by the directly inherited effect of use; and the latter, being c.u.mulative and indiscriminative in its action, would apparently have made the sole very much thicker and harder than it is. If natural selection were not supreme in such cases, how could we account for the effects of pressure resulting in hard hoofs in some cases and only soft pads in others?

A SOURCE OF MENTAL CONFUSION.

Of course in a certain sense this thickening of the sole has resulted from use. In one sense or other, most--or perhaps all--of the results of natural selection are inherited effects of use or disuse. Natural selection preserves that which is of use and which is used, while it eliminates that which is useless and is not used. The most confident a.s.sertions of the effects of use and disuse in modifying the heritable type, appear to rest on this indefeasible basis. Darwin's statements concerning the effects of use and disuse in evolution can frequently be read in two senses. They often command a.s.sent as undeniable truisms as they stand, but are of course written in another and more debatable sense. Thus in the case of the shortened wings and thickened legs of the domestic duck, I believe equally with Darwin and Spencer that "no one will dispute that they have resulted from the lessened use of the wings and the increased use of the legs." "Use" is at bottom the determining circ.u.mstance in evolution generally. The trunk of the elephant, the fin of the fish, the wing of the bird, the cunning hand of man and his complicated brain--and, in short, all organs and faculties whatsoever--can only have been moulded and developed by use--by usefulness and by using--but not necessarily by use-inheritance, not necessarily by directly inherited effects of use or disuse of parts in the individual. So, too, reduced or rudimentary organs are due to disuse, but it by no means follows that the diminution is caused by any direct tendency to the inheritance of the effects of disuse in the individual. The effects of natural selection are commonly expressible as effects of use and disuse, just as adaptation in nature is expressible in the language of teleology. But use-inheritance is no more proven by one of these necessary coincidences than special design is by the other. The inevitable simulation of use-inheritance may be entirely deceptive.

Darwin thinks that "there can be no doubt that use in our domestic animals has strengthened and enlarged certain parts, and disuse diminished them; and that such modifications are inherited." Undoubtedly "such" or _similar_ modifications have often been inherited, but how can Darwin possibly tell that they are not due to the simulation of use-inheritance by natural or artificial selection acting upon general variability? Of the inevitability of selection and of its generally adaptive tendencies "there can be no doubt," and panmixia would tend to reduce disused parts; so that there _must always_ remain grave doubts of the alleged inheritance of the similar effects of use and disuse, unless we can accomplish the extremely difficult feat of excluding both natural and artificial selection as causes of enlargement, and panmixia and selection as causes of dwindling.

WEAKNESS OF USE-INHERITANCE.

Use-inheritance is normally so weak that it appears to be quite helpless when opposed to any other factor of evolution. Natural selection evolves and maintains the instincts of ants and termites in spite of use-inheritance to a more wonderful degree than it evolves the instincts of almost any other animal with the fullest help of use-inheritance. It develops seldom-used horns or natural armour just as readily as constantly-used hoofs or teeth. s.e.xual selection evolves elaborate structures like the peac.o.c.k's tail in spite of disuse and natural selection combined. Artificial selection appears to enlarge or diminish used parts or disused parts with equal facility. The a.s.sistance of use-inheritance seems to be as unnecessary as its opposition is ineffective.

The alleged inheritance of the effects of use and disuse in our domestic animals must be very slow and slight.[51] Darwin tells us that "there is no good evidence that this ever follows in the course of a single generation." "Several generations must be subjected to changed habits for any appreciable result."[52] What does this mean? One of two things.

Either the tendency is very weak, or it is non-existent. If it is so weak that we cannot detect its alleged effects till several generations have elapsed, during which time the more powerful agency of selection has been at work, how are we to distinguish the effects of the minor factor from that of the major? Are we to conclude that use-inheritance _plus_ selection will modify races, just as Voltaire firmly held that incantations, together with sufficient a.r.s.enic, would destroy flocks of sheep? Is it not a significant fact that the alleged instances of use-inheritance so often prove to be self-conflicting in their details?

For satisfactory proof of the prevalence of a law of use-inheritance we require normal instances where selection is clearly inadequate to produce the change, or where it is scarcely allowed time or opportunity to act, as in the immediate offspring of the modified individual. Of the first kind of cases there seems to be a plentiful lack. Of the latter kind, according to Darwin, there appears to be none--a circ.u.mstance which contrasts strangely and suspiciously with the many decisive cases in which variation from unknown causes has been inherited most strikingly in the immediate offspring. It must be expected, indeed, that among these innumerable cases some will accidentally mimic the alleged effects of use-inheritance.

If Darwin had felt certain that the effects of habit or use tended in any marked degree to be conveyed directly and c.u.mulatively to succeeding generations, he could hardly have given us such cautious, half-hearted encouragement of good habits as the following:--"It is not improbable that after long practice virtuous tendencies may be inherited." "Habits, moreover followed during many generations probably tend to be inherited."[53] This is probable, independently of use-inheritance. The "many generations" specified or implied, will allow time for the play of selective as well as of c.u.mulatively-educative influences. There must apparently be a const.i.tutional or inheritable predisposition or fitness for the habits spoken of, which otherwise would scarcely be continued for many generations, except by the favourably-varying branches of a family: which again is selection rather than use-inheritance.

Where is the necessity for even the remains of the Lamarckian doctrine of inherited habit? Seeing how powerful the general principle of selection has shown itself in cases where use-inheritance could have given no aid or must even have offered its most strenuous opposition, why should it not equally be able to develop used organs or repress disused organs or faculties without the a.s.sistance of a relatively weak ally? Selection evolved the remarkable protective coverings of the armadillo, turtle, crocodile, porcupine, hedgehog, &c.; it formed alike the rose and its thorn, the nut and its sh.e.l.l; it developed the peac.o.c.k's tail and the deer's antlers, the protective mimicry of various insects and b.u.t.terflies, and the wonderful instincts of the white ants; it gave the serpent its deadly poison and the violet its grateful odour; it painted the gorgeous plumage of the Impeyan pheasant and the beautiful colours and decorations of countless birds and insects and flowers. These, and a thousand other achievements, it has evidently accomplished without the help of use-inheritance. Why should it be thought incapable of reducing a pigeon's wing or enlarging a duck's leg? Why should it be credited with the help of an officious ally in effecting comparatively slight changes, when great and striking modifications are effected without any such aid?

FOOTNOTES:

[15] Weismann's _Essays on Heredity_, &c. Clarendon Press, 1889.

[16] _Life and Letters_, i. p. 16. Darwin's reverence for his father "was boundless and most touching. He would have wished to judge everything else in the world dispa.s.sionately, but anything his father had said was received with almost implicit faith; ... he hoped none of his sons would ever believe anything because he said it, unless they were themselves convinced of its truth--a feeling in striking contrast with his own manner of faith" (_Life and Letters_, i. pp. 10, 11).

[17] _Ibid._, i. p. 38.

[18] _Life and Letters_, ii. p. 14.

[19] _Origin of Species_, pp. 117, 118.

[20] _Ibid._, p. 180.

[21] _Contemporary Review_, December, 1875, pp. 89, 93.

[22] _Variation of Animals and Plants under Domestication_, i. 292.

[23] _Variation of Animals and Plants under Domestication_, i. 299-301.

[24] To keep pace with this lateral increase in weight, the leg-bones should have lengthened considerably so that their total deficiency in proportional length is 17 per cent.,--a changed proportion which being _linear_ is more excessive than the increase of weight by 28 per cent.

So marked is the effect of the combined thickening and shortening that in the Aylesbury breed--which is the most typically representative one--the leg-bones have become 70 per cent. heavier than they should be if their thickness had continued to be proportional to their length.

[25] This excessive thickening under disuse appears to be due partly to a positive lateral enlargement or increase of proportional weight of about 7-1/2 per cent., and partly to a shortening of about 15 per cent.

Carefully calculated, the reduction of the weight of the wing-bones in this breed is only 83 per cent. relatively to the whole skeleton, or only 5 per cent. relatively to the skeleton _minus_ legs and wings. The latter method is the more correct, since the excessive weight of the leg-bones increases the weight of the skeleton more than the diminished weight of the wing-bones reduces it.

[26] _Variation of Animals and Plants under Domestication_, i. 284.

[27] _Variation of Animals and Plants under Domestication_, i. 184, 185.

[28] _Ibid._, i. 144, 145.

[29] _Ibid._, i. 185.

[30] _Variation of Animals and Plants under Domestication_, i. 175.

[31] _Variation of Animals and Plants under Domestication_, i. 184. I suspect that Darwin was in poor health when he wrote this page. He nods at least four times in it. Twice he speaks of "twelve" breeds where he obviously should have said eleven.

[32] If a prominent breast is admired and selected by fanciers, the sternum might shorten in a.s.suming a more forward and vertical position.

If the shortening of the sternum is entirely due to disuse, it seems strange that Darwin has not noticed any similar shortening in the sternum of the duck. But selection has not tended to make the duck elegant, or "pigeon-breasted"; it has enlarged the abdominal sack instead, besides allowing the addition of an extra rib in various cases.

[33] _Variation of Animals and Plants under Domestication_, 144, 175.

[34] _Variation of Animals and Plants under Domestication_, i. 179.

[35] In the six largest breeds the shortening of the sternum is nearly twice as great as in the three smaller breeds which remain nearest the rock-pigeon in size. We can hardly suppose that use-inheritance especially affects the eight breeds that have varied most in size. If we exclude these, there is only a total shortening of 7 per cent. to be accounted for.